This invention relates to the production of a constant-intensity light flux at a desired wavelength using a feedback control system which maintains the intensity of a light-producing lamp at a predetermined value over a large ambient temperature range. Such a constant-intensity source is desirable in instruments utilizing the principles of nuclear magnetic resonance such as a magnetic resonance gyroscope. In this type of gyroscope, two such light source are employed, one for the purpose of "pumping" the magnetic resonance and a second for "readout" of the phase of the resonance. It is the stability of the magnetic resonance which bears on the bias performance of the gyro. Absorbed light variations arising from intensity variations of the pump lamp, operating at a wavelength of 254 nm, produce frequency changes of the resonance causing a gyro rate bias shift. Fluctuations of intensity of the readout lamp, though not strongly absorbed, also influence the frequency of magnetic resonance likewise producing rate bias changes.